The Navy retained the name that the ship carried when she was reacquired.

Somerset (AK-212) was laid down under a Maritime Commission contract (MC Hull No. 2166) on 9 October 1944 at Sturgeon Bay, Wis., by the Leathem D. Smith Shipbuilding Co.; launched on 21 January 1945; and sponsored by Mrs. Fred Bradley. Initially earmarked to be manned by a U.S. Coast Guard crew, Somerset was completed at her building yard on 19 February 1945. After she successfully completed her Maritime Commission acceptance trials, a Navy sub-board of inspection and survey recommended preliminary acceptance on 22 February 1945.

Broken-down for the voyage via inland waterways, the ship arrived at New Orleans, La., on 2 May 1945 to be transferred to Pendleton Shipyards at New Orleans for reassembly. Accepted by the Navy on 20 September 1945 within a month of the Japanese surrender, Somerset began the conversion process to a cargo ship on 24 September. Ironically, her prospective commanding officer reported on 28 September that progress of the work was proceeding satisfactorily and that crew deficiencies caused by demobilization had been corrected, when, that same day [28 September] the ship was earmarked for return to the Maritime Commission. Her assignment to the Pacific Fleet was cancelled on 29 September. Her estimated commissioning date had been 15 October. Redelivered to the War Shipping Administration (WSA) at 1500 on 2 November 1945, Somerset was stricken from the Navy Register on 5 December 1945 never having been commissioned.

On the same day the WSA received the cargo vessel, renamed Coastal Sentry, that agency transferred her to the Stockard Steamship Company at New Orleans. Transferred again, to the War Department, on 2 August 1946 at Baltimore, Md., the ship entered the Reserve Fleet, berthed at Suisun Bay, Ca., on 28 September 1949, with the Army retaining her title. On 12 December 1949, however, Coastal Sentry was declared surplus.

The ship resumed commercial operation under the house flag of the Matson Navigation Company on 15 November 1951. Returned to the Reserve Fleet, this time at Astoria, Ore., on 17 February 1954, Coastal Sentry was taken out of reserve status on 10 May 1955 to be operated by the West Coast Trans-Oceanic Steamship Line for the Military Sea Transportation Service (MSTS). Another stint of inactivity followed (19 October 1955-22 May 1956), after which she again served under MSTS, this time with the Coastwise Line.

Placed in reserve at Olympia, Wa., on 28 September 1956, Coastal Sentry was taken out of the Reserve Fleet on 29 March 1957. Retaining her name, she was operated by the U.S. Air Force as a missile range instrumentation ship (AGM). Reacquired by the Navy on 1 July 1964 and reinstated on the Naval Vessel Register, Coastal Sentry was designated as T-AGM 15.

Along with Rose Knot (T-AGM 14), Coastal Sentry not only collected and relayed radio telemetry information from on board spacecraft, but operated in the command-control role, having embarked National Aeronautics and Space Administration (NASA) flight controllers. As an example of that work, Coastal Sentry initiated the necessary re-entry command signals on the Gemini VIII mission when the capsule had to make an emergency landing in the Pacific on 17 March 1966.Transferred to the Maritime Administration (the successor of the Maritime Commission) on 11 July 1968, the ship was delivered the same day to the firm of Fuji Marden and Co., Ltd., of Hong Kong, British Crown Colony, at Fremantle, Australia, for scrapping. Coastal Sentry (T-AGM 15) was stricken from the Naval Vessel Register on 9 October 1969.